t*|£ 


,       GIFT   OF 
*  " 


The  Bridge 


THE 

CONCRETE  BRIDGE 


A  book  on  why  the  Concrete  Bridge  is  replacing  other 
forms  of  Bridge  Construction 


By 

JNO.  B.  LEONARD  and  W.  P.  DAY 

If.    Am.   Sor.    C.    E.  AlMC.    M.   Am.   .Sor.    C.    E. 

SAN  FRANCISCO 


COPYRIGHT  1913 

BY 

LEONARD     &     DAY 

CONSULTING  ENGINEERS 

RIALTO  BUILDING,  SAN  FRANCISCO 

PRICE,  $1.00 


Gift  of 


PREFACE 

The  purpose  of  this  publication  is  to  supply  information  as  to  the  vast  strides 
made  in  California  during  the  past  few  years  in  the  use  of  reinforced  concrete  for 
permanent  bridge  construction.  It  is  believed  that  there  exists  in  the  minds  of  the 
public  a  limited  understanding  of  what  has  been  accomplished  in  this  direction. 
I  strong  conviction  is  growing  to-day,  however,  in  favor  of  the  desirability  of 
structures  of  this  character,  due  to  the  satisfaction  given  by  those  in  existence. 

The  publishers    realize    that  they  have  only  touched  briejly  upon  the  subject. 
Through  their  connection  with  all  of  the  work  shown  herein,  they  are  in  possession  of 
tletailed  knowledge  of  both  the  design  and  construction  of  each.      They  invite  and 
will  cheerfully  answer  any  inquiries  addressed  to  them  for  further  information. 

JNO.  B.  LEONARD. 
W.  P.  DAY. 

l-'i-l>ruary  20.  1913. 
528-530  Riallo  Building. 
San  Francisco,  California. 


264233 


/  •     '' 

( 

'Conformity  with  environment,  economic  use  of  material,  pleasing  outline  and  appropriate  use  of  ornament 
make  toward  beautiful  bridges,  which  are  a  sure  indication  of  a  progressive  community." 


0  0  C  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0 


rzi 


Why  it  is  the  bridge 


|HE  USE  OF  REINFORCED  CONCRETE  for  the  construction  of 
bridges,  in  the  State  of  California,  has  increased  during  the  last  few 
years  in  a  very  marked  degree.  With  the  advance  in  scientific  infor- 
mation as  to  the  action  of  steel  and  concrete  in  supporting  loads, 
designers  and  builders  have  advanced,  step  by  step,  until  today  long 
spans  and  important  structures  are  entirely  feasible  in  the  hands  of 
competent  engineers  and  constructors.  As  a  result,  many  highway 
bridges  throughout  the  State  are  built  of  reinforced  concrete,  this  type 
taking  the  place  of  the  old  combination  structures  of  steel  and  timber.  Nor  is  the  use  of 
the  material  in  this  State  confined  to  highway  bridges.  There  are  a  few  carrying  heavy 
interurban  cars,  and  the  largest  and  heaviest  locomotives. 

In  general,  there  are  two  types  of  reinforced  concrete  bridges — the  flat  span  and  the 
arched  span.  Each  type  lends  itself  to  variation,  in  accordance  with  the  ideas  of  the 
designer.  The  flat  span  is  sometimes  built  without  girders,  more  often  with  girders  run- 
ning the  long  way  of  the  span.  The  arched  span  is  sometimes  built  as  a  continuous  arch 


Page    Seven 


EASE  OF  CONSTRUCTION 


u 

u 

u 

u 

u 

u 

u 

0000000000000000 

u  u 

eg 

with  side  walls  and  earth  roadway.  Occasionally  the  side  walls  are  omitted,  and  the 
roadway  is  carried  on  columns  resting  on  the  arch.  The  arch  is  not  always  the  full  width 
of  the  bridge,  the  width  being  made  up  of  two  or  more  individual  arches  with  a  light  floor 
between. 

It  is  interesting  to  know  that  reinforced  concrete  construction  tends  toward  the  use 
of  our  home  products.  Timber  for  temporary  supports  and  moulding  forms  is  easily 
obtained  on  our  own  coast;  cement  and  reinforcing  steel  are  manufactured  in  our  own 
State;  sand  and  gravel  are  obtained  usually  on  the  site  of  the  bridge,  or  at  least  not  very 
far  away;  our  rock  quarries  produce  an  abundance  of  crushed  rock,  where  it  is  desirable 
to  use  this  material.  Annoying  delays  that  often  occur  in  the  building  of  steel  bridges 
are  avoided  by  reason  of  the  fact  that  cement  and  reinforcing  steel  are  manufactured  in 
California. 

Methods  of  construction  vary,  of  course,  with  the  design  and  the  location.  Forms 
and  temporary  supports  for  flat  span  bridges  offer  no  particular  difficulty.  Temporary 
supports — generally  called  falsework — for  arches  must  be  carefully  designed  and  executed. 
Usually  there  is  water  under  the  arch,  and  piles  are  driven  to  carry  the  posts  of  the  false- 
work. The  proper  time  to  begin  construction  is  the  low  water  period.  This  gives  the 
contractor  ample  time  during  the  spring  season  to  place  his  plant  on  the  site,  and  make 
general  preparation  for  the  work.  Extreme  care  should  be  used  in  the  proportioning  of 
the  concrete  mixture,  as  well  as  in  the  method  of  casting.  The  ideal  arch  is  that  in  which 
the  work  is  carried  on  so  continuously,  that  no  joints  occur  as  a  result  of  the  previous  work 


1'age    Eight 


DURABILITY 


1 

I) 

U 

u 

U 

u 

0000000000000000 

u 

u 

u 

u 

ED 

drying  out  before  the  next  can  take  hold,  and  yet  not  so  fast  but  that  the  regular  shrinkage 
in  setting  has  been  allowed  for.  Contractors  usually  prefer  to  cast  side  walls  before  remov- 
ing the  falsework,  but  this  is  apt  to  produce  cracking  in  the  walls,  which,  while  not  dangerous, 
is  unsightly.  The  proper  method  is  to  allow  the  arch  to  take  its  natural  form  before  casting 
the  walls.  All  arches  drop  at  the  crown  when  the  falsework  is  removed,  owing  to  the 
compression  of  the  arch  under  its  own  weight.  The  probable  drop  at  the  crown  may  be 
very  closely  calculated  and  allowed  for  in  the  forms,  in  addition  to  the  allowance  for  the 
compression  of  the  falsework  timbers. 

The  element  of  cost  is  naturally  of  first  importance  to  the  prospective  builder,  and  is 
worthy  of  careful  consideration.  In^the  first  place,  it  is  generally  understood  that  a 
concrete  bridge  is  more  costly  than  a  steel  bridge.  This  statement,  needs  explanation  in 
order  to  convey  the  truth.  It  so  happens  that  a  concrete  bridge  may  be  cheaper  than  a 
steel  one,  depending  entirely  upon  the  live  or  moving  load.  For  example,  it  is  cheaper  to 
build  a  reinforced  concrete  bridge,  to  carry  locomotive  and  train  loads,  than  it  is  to  build 
a  steel  bridge  for  the  same  loads.  On  the  other  hand,  a  light  highway  bridge  of  steel  with 
timber  floor  is  cheaper  than  a  concrete  one.  Concrete  bridges  are  so  heavy  in  themselves 
that  the  addition  of  a  heavy  live  load  does  not  materially  alter  their  design.  With  the 
steel  bridge,  the  live  load  is  the  all  important  factor,  a  heavy  live  load  necessitating  a  very 
much  heavier  structure,  as  well  as  more  costly.  Concrete  bridges  designed  to  carry  loco- 
motive loads  are  at  least  15  per  cent  cheaper  than  steel  bridges  designed  for  the  same 
loading.  The  only  type  of  steel  highway  bridge  that  is  cheaper  than  a  concrete  bridge  is 


Page    Nine 


PERMANENCY 


0000000000000000000000000 


that  with  a  timber  floor.  Timber  floors  wear  rapidly  and  must  be  teplaced  about  once 
in  three  years,  depending,  of  course,  on  the  traffic.  Under  the  usual  conditions,  if  a  steel 
bridge  be  built  with  a  solid  concrete  floor  to  avoid  the  replacing  of  the  timber,  the  cost  of 
the  structure  will  equal  that  of  a  concrete  bridge.  It  is  a  well-known  fact  that  all  steel 
bridges  must  be  painted  periodically  in  order  to  preserve  them.  It  is  equally  well  known 
that  after  forty  or  fifty  years  at  the  most,  the  steel  has  deteriorated  to  such  an  extent  as 
to  make  replacement  of  the  structure  a  necessity.  It  must  be  borne  in  mind,  also,  that 
county  bridges  will  not  last  as  long  as  railroad  bridges,  because  the  former  are  not  given 
the  same  care  as  the  latter.  It  may  be  seen,  therefore,  that  steel  bridges  require  main- 
tenance, and  that  further,  they  have  a  limited  existence.  Concrete  bridges  require  abso- 
lutely no  maintenance,  and  the  effect  of  age  is  to  strengthen  rather  than  weaken. 

From  the  foregoing,  it  is  manifestly  impossible  to  set  a  definite  ratio  of  cost  between  a 
concrete  and  steel  bridge.  Cost  is  always  a  question  of  locality,  of  availability  of  materials, 
and  of  the  foundations,  all  of  which  vary  in  each  case.  Steel  bridges  are  sometimes,  in 
fact  often,  built  on  cylinder  piers,  which  cannot  compare  as  supports  with  the  solid  piers 
used  for  concrete  bridges.  Assuming,  however,  that  a  concrete  highway  structure  for  a 
given  locality  can  be  built  for  $100,000,  we  may  safely  say  that,  ordinarily,  a  steel  bridge, 
with  timber  floor,  can  be  built  for  $85,000,  and  that  its  life,  with  proper  maintenance,  is 
fifty  years.  On  this  basis  the  annual  charge  for  the  steel  bridge  would  be  about  as  follows: 


Page     Ten 


OW        UP-KERP 


cn 

I) 

0 

0 

0 

0 

0 

0 

0 

00000000000000000 

ED 

Average  annual  charge  for  repainting,  assuming  the  bridge  is 

painted  once  in  5  years  at  a  cost  of  $3,000 $    600.00 

Average  annual  charge  for  renewing  floor 400.00 

Annual  interest  charge  at  4  per  cent  on  the  investment 3,400.00 

Annual  sinking  fund  for  renewal  at  end  of  50  years  at  4  per  cent 

compounded 556.75 

Total  annual  charge $4,956-75 

The  only  charge  against  the^concrete  bridge  is  the  annual  interest  on  the  investment, 
which,  at  4  per  cent,  amounts  to  $4,000,  or  a  net  saving  of  $956.75  per  year  in  favor  of  the 
concrete  bridge. 

In  addition  to  the  advantage  shown  in  the  preceding,  in  favor  of  the  concrete  bridge, 
there  is  the  artistic  difference  between  the  two,  for  which  no  monetary  value  may  be 
assigned.  The  artistic  side  of  bridge  building  has  been  given  less  attention  than  its  impor- 
tance warrants.  Concrete  lends  itself  beautifully  to  this  phase  of  design.  The  Oakland 
Avenue  Bridge,  shown  on  pages  12-13,  was  designed  with  a  view  to  pleasing  appearance, 
and  exemplifies  the  ability  of  concrete  to  meet  this  condition. 


Page    Eleteti 


THE  CONCRETE  BKIDQE 


Showing  the  possibility  for  artistic  treatment  and  suggestive  of  the  adaptability  of  reinforced  concrete 

for  city,  suburban  or  park  bridges 

Portals  Oakland  Avenue  Bridge 
Piedmont,  California 


Page    Twelve 


THE  CONCRETE  BRIDGE 


Practical  and  picturesque,  thin  hridffc  i.s  an  excellent  example  of  ornnmental  'concrete  cousiruciinn   und  its 

power  to  enhance  the  natural  surroundings 
Oakland  Avenue  Bridge 
Piedmont,  California 


CLEAR  SPAN  OF  130  FEET 

LENGTH  OVER  ALL  363  FEET 

ROADWAY  22  FEET  WITH 

Two  6  FOOT  OVERHANGING  SIDEWALKS 

BUILT  1911 


Page     Thirteen 


THE  CONCRETE  BRIDGE 


A  happy 


and  niix/cni   i>n«in<'<-riii£  i<li'ds.   lliis  Itriiltif.    ii'ilh   its 
'<:  ini-ol\-<-d  many  problems  in  its  construction 

A  view  of  approach,  showing  ornamental   kiosk 
Oakland  Avenue  Bridge 
Piedmont,  California 


Page  Fourteen 


THE       'CONCRETE  BRIDGE 


Note  the  pleasing  spring  of  tin1  urcli.  uh 


Dry  Creek  Bridge 
Stanislaus  County,  California 

CLEAR  SPAN  OF  112  FEET 
LENGTH  OVER  ALL  160  FEET 
ROADWAV  2(i  FEET  CLEAR 
BUILT  1907 


Page    Fifteen 


THE  CONCRETE  BRIDGE 


bridges  designed  strictly  on  the  cantilever  principle,  making 
depth  in  the  carrying  girders 

Ross  Bridge  No.  1 
Ross  Valley  Station 
Marin  County,  California 


Fagf    Sixteen 


MAIN  SPAN  45  FEET 

END  SPANS  20  FEET 

LENGTH  OVER  ALL  87  FEET 

ROADWAY  38  FEET 

BUILT  \'M> 


THE     CONCRETE     BRIDGE 


Of  simple  design  and  economical  construction,  it   is  readily  seen   that  it   Inld^e  i>/' 

be  made  an   ornament  in  similar  locations 
Ross  Bridge  No.  3 


type  can 


MAIN  SPAN  22  PEET 
END  SPANS  10  FEET 
LENGTH  OVER  ALL  43  FEET 
ROADWAY  20  FEET  IN  THE  CLEAR 
BUILT  1909 


Page    Seventeen 


THE     CONCRETE     BRIDGE 


. 


^romantic  countryside.     The  concrete,  blending  with 
offers  no  harsh  contrast  even  when  new 


Page    Eighteen 


Ross  Bridge  No.  2 


CLEAR  SPAN  48  FEET 

LENCTH  OVER  ALL  81  FKKT 

ROADWAY  20  FKKT  IN  THE  CLEAR 

BUILT  I'.NK.I 


HE  CONCRETE  BRIDGE 


of  Marin  County's  artistic  bridgi-s:   of  the  romrr/r  .^i /•(/<•/•  tvp<-.  <int/>!< 


Bridge  at  San  Anselmo 
Marin  County,  California 

CLEAR  SPAN  46  FEET 
ROADWAY  40  FEET 
LENGTH  OVER  ALL  80  FEET 
BUILT  1910 


in  design 


Page    N  ineteen 


One  of  the  longest  concrete  bridges  on  the  Pacific  Coast.     Each  pier  rests  on  250  piles  and  is  of  extremely  masxi 


Eel  River  Bridge 

Near  Eureka 

Humboldt  County,  California 


Page    Twenty 


THE  CONCRETE  BRIDGE 


tion,  to  withstand  the  heavy  drift.     Eel  River  has  a  rapid  current  and  is  often  filled  with  drifting  saw  logs 


SEVEN  200  FOOT  SPANS 
LKNI;TH  OVER  ALL  1451  FEET 
ROADWAY  22J  FEET  CLEAR 
BUILT  1911 


Page    Twenty-one 


Greeted  jointly  l>\  l-'resnn  <in<l    Matlem"  ('oimti'-s.  itn/1  limit  largely  through  the  confidence  created  in  favor  of 

concrcti}  '•»nxiru<-tion   In-  the  erection  of  the  Pollasky  bridge 

Skaggs  Crossing  Bridge 

San  Joaquin  River 

Between  Fresno  and  Madera  Counties,  California 


FIVE  SPANS  OF  80  FEET  EACH 

20  FOOT  CLEAR  ROADWAY 

BUILT  1907 


Page     Twenty-two 


THE  CONCRETE  BRIDGE 


Erected  jointly  by  Fresno  and  Modern  ('.ouniifs.      'I  In-  siujfft 

of  the  largest  in  California,  led  to  enae  of  ronslrm  linn   mnl  ronseijiient 
Pollasky  Bridge 
Over  San  Joaquin  River 
Between  Fresno  and  Madera  Counties.  California 


.siYc  structure,  one 


TEN  75  FOOT  SPANS 
LENGTH  OVER  ALL  780  FEET 
ROADWAY  Is  FEET  CLEAR 
UI-II.T  iwii 


Page    T-u'enty-thrre 


THE 


CONCRETE 


BRIDGE 


combinatu 


the  roadway  and  flume.     Note  volume  of  water  in  the  flume 

Weir  across  Temple  Slough  at  its  junction  with 

San  Joaquin  River,  Merced  County,  California 

Built  for  Miller  &  Lux,  Inc. 


LENGTH  OVER  ALL  83  FEET.  6  INCHES 

CARRYING  ROADWAY  14  FEET  WIDE 

FLUME  4  BY  20  FEET 

BUILT  1911 


Page    Twenty- four 


THE     CONCRETE     BRIDGE 


Another  view  of  the  Temple  Slough  weir.     The  nxid^tn  perfurms  the  'e  function  <> 

its  ireifflil.  milling  In  /In'  stability  of  the  ireir 

Operated  as  easily  as  a  timber  structure.     Not  subject  to 
deterioration,  but  increasing  in  strength  with  its  age 


Inijfir  nnil.  through 


rage    Twenty-five 


THE 


CONCRETE 


B    R 


D    Q    E 


Thix  coinliinatiim   hrirli. 


nifl 


*li>n<>li   irhirh  controls  the  water  from  Buena   Vista  Lake 


Old  Headquarters  Weir 

Miller  &  Lux,  Inc. 

Kern  County,  California 

LENGTH  OVER  ALL  16.3  FEET 

ROADWAY  13  FEET  IN  THE  CLEAR 

HKIGHT  FROM  FLOOR  OF  WEIR  TO 

lionoM  (IK  HKIIM;E  SLAB  19  FEET 

BUILT  1911 


Page     Twenty-six 


THE     CONCRETE     BRIDGE 


Another  view  of  the  Old  Headquarters   Weir,  showing  the  ro<nlxti\    anil  ul   the  left  a  concrete  sluh  that 

can   be  used  for  a  footicalk 

The  maintenance  expense  and  necessary  renewals  of  the 
timber  weir  which  was  replaced  by  this  substantial 
structure  proved  the  economy  of  the  latter 


Page     Twenty-seven 


THE  CONCRETE  BRIDGE 


Tin'  i-diisirnciinn   here  necesxitdted  it  full  consider! 1 1  ion  of  the  rapid  current  of  the  river  and  its  heavy 

drift   of  ice  in    irinlcr     M  <-/j    in   I  lie  illiislriiiinn 

Virginia  Street  Bridge 
Truckee  River 
Reno,  Nevada 

SHOWING  THE  UNUSUAL  WIDTH  OF  ROADWAV,  WHICH  is  EQUAL  TO  THAT 

OF  THE  AVENUE  ON  WHICH  IT  is  LOCATED 

ROADWAY  AND  SIDEWALKS  80  FEET 


Page     Twenty-eight 


THE          CONCRETE  BRIDGE 


This  is  one  of  the  first  bridges  built  on  the  Pacific  Const  of  rcinforci'il  concrete     a  pioneer.     The  only 

bridge  shown,  not  in  California 

Profile  view  of  Virginia  Street  Bridge,  Reno,  Nevada, 
Showing  it  as  it  appears  to  the  casual  observer 


Two  65  FOOT  SPANS 
BUILT  1905 


Page    Twenty-nine 


THE  CONCRETE  BRIDGE 


/    /.•>   <l<'f>r<'f 


Inulfre.  llic 
the 


tl  meeting  each   cud  on   a    l(>  degree  curve.     Built  on  solid  rock  and 
locornotirrs  hiinlinf!  t-urs  ln-nvily  laden  with  limestone 


American  River  Bridge 

Near  Auburn 

Placer  County,  California 

THREE  140  FOOT  CLEAR  SPANS 

LENGTH  OVER  ALL  652  FEET 

BUILT  1011 


Page     Thirty 


THE  CONCRETE  BRIDGE 


This  bridge  stands   75  feet  above  the  river,  anil  the  sJn-ir  or  ii 

seen.      Note  the  heavy  Mallei  compound  engine,   intliciiti 

East  approach  of  American  River  Bridge 

Near  Auburn, 

Placer  County,  California 

With  birdseye  view  of  river 


?  crosses  the  river  is  here  plainly 
nd  upon   the  structure 


Page    Thirty-one 


THE  CONCRETE  BRIDGE 


The  cnri-c  of  this 


e  for  flood  water,  while  conforming 


Essex  Street  Bridge 
San  Luis  Obispo 

CLEAR  SPAN  60  FEET 

LENGTH  OVER  ALL  120  FEET 

BUILT  19011 


Page    Thirly-lwo 


THE     CONCRETE     RR1DQE 


An  example  of  the  concrete  girt 

low 

Marsh  Street  Bridge 
City  of  San  Luis  Obispo 
San  Luis  Obispo  County,  California 


CLEAR  SPAN  OF  40  FEET 
LENGTH  OVER  ALL  80  FEET 
BUILT  1909 


Page    Thirty-three 


THE  CONCRETE  BRIDOE 


llnili  to  replace  an  old  type  combination  structure  of  steel  and  wood.     Notable  for  extremely  low  cost  of  construction 

Lindo  Channel  Bridge 
Chico 
Butte  County,  California 


Two  SPANS,  EACH  87  FEET  CLEAR 

ROADWAY  28S  FEET  CLEAR 

LENGTH  OVER  ALL  204  FEET 

BUILT  1912 


Pngt    Thirty-four 


THE     CONCRETE     BRIDGE 


Situated  far  up  in  the  mountains,   this  structure  is  interesting  lin~i7irSr~j,  ;7n>wr  depth   excarnted  to 

reach  bedrock,  the  bottom  of  the  mid-river  filer  bcin/s  nearly^SO  fri't  beloir  the  tenter 

Bridge  over  Stanislaus  River 

Between  Stanislaus  and  Tuolumne  Counties,  California 


Two  100  FOOT  SPANS  AND  ONE  50  FOOT  SPAN 
LENGTH  OVER  ALL  350  FEET 
BUILT  1909 


Pate    '1'hirly-five 


This 


-   Riri'r  iit'iir  Modesto,  connecting  San  Joaquin 
islftux  Counties 


Ripon  Bridge 

Two  110  FOOT  SPANS 

LKNUTH  OVER  AM.  280  FEET 

BUILT  1908 


I'age     Thirty-six 


THE  CONCRETE  BRIDGE 


This  is  an  original  design   «rr<y)/<></  /n    lln-   Luiiilsrti/x'   l-'.n<rin<-cr. 

its  lines  were  in  complete  accord  n-ith  its  siirrouni  -(instructed  l>v  tht 

Peninsular  Railway  ('.OHI/HIIIY  to  carry  lln'ir  hi'iirv  inlcritrban  cars 
Alum  Rock  Bridge 
San  Jose,  California 


decided  that 


CLEAR  SPAN  130  FEET 
APPROACH  SPANS  40  FEET 
LENGTH  OVER  ALL  250  FEET 
BUILT  1913 


Thirty-seven 


CONCRETE 


This  bridge  has 


and  ifht'ii  completed  will  be  1100  feet  long.     Designed 
<d  remarkable  for  its  low  cost 

Trestle  Bridge 
Maxwell  and  Colusa  Road 
Colusa  County,  California 


ALL  SPANS  20  FEET 
TOTAL  LENGTH  ON  COMPLETION  1100  FEET 


Page    Thirty-eight 


THE  CONCRETE  BRIDGE 


a 

I) 

0 

0 

0 

0 

0 

0 

0  0 

0 

ooooooooooooooo  tai1 

STRAIN    SHEET 

The  stress  diagram,  erroneously  but  universally  called  strain  diagram,  should  be  made 
for  all  arches,  else  the  stresses  in  the  materials  are  not  definitely  known.  The  Elastic 
Theory  for  fixed  end  arches  is  recognized  by  most  engineers  as  being  the  best  analysis  for 
structures  of  this  type.  Stresses  should  be  obtained  for  dead  and  live  loads,  rise  or  fall 
of  temperature,  and  shortening  of  the  arch  from  superimposed  loads.  In  providing  concrete 
and  reinforcing  steel  to  meet  these  conditions,  that  combination  of  loading  which  produces 
maximum  stresses  in  the  arch-is  used. 

Choice  of  the  shape  of  the  arch  is  a  matter  of  vital  importance  from  the  standpoint  of 
economy,  although  this  fact 'is  not  generally  recognized.  A  slight  change  in  the  shape 
produces  material  changes  in  'the  stresses.  The  matter  is  complicated,  by  the  fact  that  no 
known  law  exists  by  which  the  proper  curve  may  be  determined  without  a  cut  and  try. 
The  engineer  must  rely  solely"  upon  his  experience  in  his  first  choice  'of  curve.  This  fact 
was  well  demonstrated  in  the  case  of  the  Eel  River  Bridge, "a  cut  of  which,  is  shown  on 
pages  20-21. 

The  designers  made  three  different  curves  of  the  same  span  and  same  rise  before 
obtaining  the  one  which  they  considered  correct.  For  each  individual  arch  with  a  given 
rise  and  span,  there  exists  a  curve  that  produces  minimum  stresses,  and  it  is  the  function 
of  the  designer  to  obtain  this  curve. 


Pair    Thirty-nine 


THIS  BOOK  IS  DUE  ON  THE 
STAMPED  BELOW 


AN  INITIAL  PINToP  25 


264233 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


' 


